Batch for producing a refractory ceramic shaped body, shaped body made therefrom, and a use thereof

ABSTRACT

The invention relates to a composition for the production of a refractory ceramic moulded body, a non-fired or fired moulded body formed from the composition and a possibility for use.

The invention relates to a composition for the production of arefractory ceramic moulded body, a non-fired or fired moulded bodyformed from the composition and a possibility for use.

Refractory ceramic products are subdivided into basic and non-basicproducts, for example. The basic products include those based on MgO(magnesia) such as magnesia products or magnesia-chromite products.

Such, in particularly purely magnesitic, types exhibit an excellentabrasion resistance, frequently however, an unsatisfactory infiltrationbehaviour vis-à-vis metallurgical slag such as those typical inprocesses for the production of stainless steel, for example. Suchprocesses are known as AOD (=argon-oxygen decarb processes) or VOD(=vacuum-oxygen decarburisation processes). The detaching behaviour,too, is unsatisfactory.

Apart from these purely magnesitic types, fired (including carbon-bound)bricks are known which contain at least a considerable proportion ofdolomite [CaMg(CO₃)₂] in the fired form. Depending on the deposit, thecontent of MgO or CaO of the dolomite varies. In terms of order ofmagnitude it amounts to 60% by weight CaO and 40% by weight MgO. SiO₂,Fe₂O₃, Al₂O₃, MnO (<3% by weight) in the form of their calcium compoundsare present as secondary components. Routschka “Feuerfeste Werkstoffe”(refractory materials), ISBN 3-8027-3144-1), section 4.2.6.1 suggests atypical SiO₂ content of 0.5 to 1.5% by weight, a typical Fe₂O₃-contentof 0.5 to 1.0% by weight and a typical Al₂O₃ content of 0.2 to 0.8% byweight.

Depending on whether the compositions consist exclusively of dolomite orpredominantly of dolomite (apart from additions of magnesia), themoulded parts formed therefrom are referred to as dolomite products ormagdol products. Dolomite as a component of the composition meanscalcined dolomite or sintered dolomite, i.e. components containingCaO+MgO, in each case.

The wear of a dolomite or magdol brick is considerably greater than inthe case of a pure magnesia brick. Such moulded parts containingCaO+MgO, however, have a substantially lower infiltration tendency anddetach themselves less easily.

In DE 100 10 918 A1, a composition for the production of a refractoryceramic moulded body is disclosed which comprises the followingindependent components:

-   a) 80 to 97% by weight of fused magnesite, sintered magnesite or    mixtures of these with an MgO content of >93% by weight and a grain    size of <8 mm,-   b) 3 to 20% by weight of CaO in a grain fraction of <1 mm.

An essential aspect is that calcium oxide is added as an independentcomponent of the magnesitic main component.

The moulded parts formed from this composition represent a quasicompromise between the known magnesia and magdol products. When usingcorresponding products, the expected improvement in the resistance toabrasion and the lower infiltration tendency is achieved; however,detachments can also take place.

The invention is based on the task of improving the moulded bodies knownfrom DE 100 10 918 regarding their tendency to detach themselves withouthaving to forego the good abrasion and corrosion resistance properties(as in the case of purely magnesitic moulded bodies) and the resistanceto infiltration.

The invention is based, in this connection, on the followingconsiderations: the side of a refractory ceramic moulded body which isfacing the fire (the melt) is at a particular risk of detaching itself.This side is exposed to the highest temperatures (partially of more than1,700° C.). Consequently, it was the aim of the development to improvethe properties of the product regarding its softening under pressure(according to DIN-EN 993-8, 1997) and to make the structure moreflexible. This is achieved by the following means. Small proportions ofmolten phase are admitted without negatively influencing the refractoryproperties. In this way, the thermal expansion, in particular, of MgOcan be compensated. Thermomechanical tensions are avoided. Detachments(so called “spalling”) can be prevented or at least reduced.

Consequently, the invention deviates from the teaching of DE 100 10 918A1 of adding CaO as an independent component in maximum purity to thecomposition. Instead, a CaO-containing component is used in thecomposition in a controlled manner, which component is capable ofintroducing different foreign oxides, such as Fe₂O₃, into thecomposition. In this way, dicalcium ferrite, among other things, isformed as secondary phase during firing of moulded body produced fromthe composition. Dicalcium ferrite provides the moulded body at elevatedtemperatures (application temperature) with a certain structuralelasticity such that stresses can be better absorbed and/or reduced.Moreover, iron oxide acts as a mineraliser when firing brick.

According to the invention, the iron oxide content (of the composition)can be between 1 and 8% by weight.

Consequently, the invention deviates deliberately from the requirementof the state of the art of taking into account as little Fe₂O₃, in anycase <1.0% by weight, in the composition.

One possibility of adjusting the Fe₂O₃ content consists of usingdolomite rich in iron oxide, for example. In this way, a proportion ofMgO is introduced into the composition simultaneously. A furtherproportion of MgO is provided by a purely magnesitic component, e.g.fused magnesia or sintered magnesia.

A further essential differentiation criterion with respect to thecomposition according to DE 100 10 918 A1 consists of the selection ofthe grain sizes for the individual components. Whereas, in the state ofthe art, the CaO-containing component is to be used in a grain fractionof <1 mm, both the component containing MgO and that containing CaO inthe case of the invention can be present in the composition in a grainsize of <8 mm. According to one embodiment, the CaO-containing componentcan have a grain size of >2 mm and/or <5 mm. This does not preclude thepossibility of using also a CaO-containing component or CaO fractionwith a proportion of fine grains of <1 mm or even <0.3 mm. This can be acomponent of the composition which is independent of the coarsecomponent (>2 mm).

For the MgO-containing component, insofar as it has not already beentaken into account as dolomite, the grain sizes are in particular in theregion of <4 mm. For example, ⅕ to ½ of this MgO component can be <0.3mm, the remainder >0.3 mm.

In its most general embodiment, the invention relates to a compositionfor the production of a refractory ceramic moulded body which comprisesat least one component containing MgO and CaO in a grain size of <8 mmand has the following oxide analysis:

-   a) 50 to 90% by weight of MgO-   b) 8 to 40% by weight of CaO,-   c) 1 to 8% by weight of Fe₂O₃,-   d) up to 10% by weight of others.

The sum total of a) to d) should be 100% by weight. Any binder, wateretc. is calculated separately.

The moulded parts formed from this composition can be classified asmagnesia products comprising additions containing CaO and Fe₂O₃ from thecomposition, these additions providing the fired product with propertiesthat have previously been obtained only with products having a high CaOcontent. Moreover, it is possible to achieve excellent corrosionproperties with moulded parts made from this composition, as in the caseof pure magnesia bricks. These properties are combined with a goodinfiltration resistance and improved structural elasticity such as theyhave been known previously only in the case of purely dolomitic mouldedbodies. All moulded parts such as bricks, panels, rings etc. are mouldedbodies.

The attached illustration shows a polished face of a brick according tothe invention in the indicated magnification.

The structure of the brick is determined by coarse dolomitic grains (1)with an Fe₂O₃ content of approximately 3% by weight. Between thesecoarse (in the polished face: dark) grains, comparatively smaller MgOgrains (2) can be discerned between which MgO—CaO melt additions (3) canbe discerned.

The product illustrated which was fired at 1,550° C. has the followingcharacteristic values:

Raw density DIN EN 993-1: 1995 g/cm² 3.05 Open porosity DIN EN 993-1:1995 Vol. % 13 Gas permeability DIN EN 993-4: 1995 nPm 4 Compressivestrength DIN EN 993-5: 1998 MPa 70 in the cold Softening under DIN EN993-8: 1997 ° C. 1650 pressure T_(o, 5)

An example of the composition is:

Sintered magnesia ¹⁾ (0.3–4 mm) 33% by weight Sintered magnesia ¹⁾ (<0.3mm) 12% by weight CaO + MgO sintered material ²⁾ (2 to 5 mm) 35% byweight CaO + MgO fused material (<0.3 mm) 30% by weight ¹⁾ with 96% byweight of MgO ²⁾ with 41% by weight of CaO and 3.8% by weight of Fe₂O₃.

The total MgO content is approximately 71% by weight, the total CaOcontent approximately 26% by weight, the total Fe₂O₃ contentapproximately 1.6% by weight.

The bricks fired from this working material mixture (composition) at1,400° C. have a value of T_(0.5) of 1,520° C. and an excellentresistance to detaching.

As detailed above, the MgO-containing component can consist of sinteredmagnesia, for example, with a grain fraction of <5 mm. A portion of theMgO is provided by a coarse grain of sintered dolomite in a fraction of2 to 8 mm.

The proportion of MgO and CaO can also be introduced into thecomposition via a so called MgO+CaO molten material (co-smelter) [(3) inthe illustration of the polished phase].

Insofar as no sintered dolomite is available in order to achieve therequired proportion of Fe₂O₃. in the composition, the iron oxide can beadmixed by foreign components, e.g. in the form of scale.

As a rule, the iron oxide content will be >1.4% by weight, e.g. 1.5 to2% by weight, however, it can also be adjusted to values of >2% byweight, e.g. 2 to 4% by weight, an upper limit of 3% by weight beingfrequently sufficient to achieve the desired structural flexibility. Thestructural flexibility can also be characterised as follows:

The test for softening under pressure according to DIN EN 993-8 (1997)provides T_(0.5) values of between 1,400° C. and 1,700° C., valuesbetween 1,500° C. and 1,650° C. being advantageous.

The other foreign oxides such as Al₂O₃, MnO and SiO₂ can be adjusted tovalues of <2 or <1% by weight, in each case.

The MgO-containing component, insofar as it is introduced as puremagnesitic component, should have a degree of purity of >90% by weight,in particular >95% by weight.

The mean grain size (d₅₀) of the CaO-containing component can beselected to be larger than the mean grain size (d₅₀) of theMgO-containing component, the MgO-containing component being intended tohave a degree of purity of >90% by weight, in particular >95% by weight.

According to one embodiment, the above-mentioned ratio also appliedregarding a grain size of “d₉₅” in each case.

Within the oxide analysis indicated, non-fired ceramic moulded bodiescan be produced using the above-mentioned components containing MgO andCaO, a binder being usually admixed to the composition. The binder canbe a carbon-containing temporary binder such as paraffin, for example.

From this non-fired product, a fired moulded body can be produceddirectly, the firing process taking place in a standard oven attemperatures above 1,400° C.

The components of the composition, their grain size and the firingtemperature can be selected in such a way that the fired moulded bodyhas a raw density of >3 g/cm³. The raw density leads to a relatively lowopen porosity which, according to one embodiment, is indicated as being<14% by volume, values of <13.5 or <13% by volume being aimed at.

The porosity and raw (apparent) density is responsible for the requiredinfiltration resistance. The products have a good resistance todetaching and a high resistance to corrosion. They are also suitable fordifficult fields of application in steel manufacture and in rotaryovens, e.g. for the production of cement. The content of Fe₂O₃ can alsobe above 4% by weight, e.g. 6 or 8% by weight, the structuralflexibility being further increased at low application temperatures.

1. A fired ceramic moulded body, made of a composition which comprisesat least one purely magnesitic component and at least one componentcontaining CaO, all in a grain size <8 mm, and which composition has thefollowing oxidic analysis: a) 50 to 90% by weight MgO, b) 8 to 40% byweight CaO, c) 1 to 8% by weight Fe₂O₃, d) up to 10% by weight others,the sum total of a) to d) being 100% by weight, the moulded body havinga test value T_(0.5) according to DIN EN 993-8 (1997) of between 1,400and 1,700° C., and comprising dicalciumferrite.
 2. Moulded bodyaccording to claim 1 with a density of >3 g/cm³.
 3. Moulded bodyaccording to claim 1 with an open porosity of <14% by volume.
 4. Mouldedbody according to claim 1 in which the purely magnesitic component has adegree of purity of >90% by weight MgO.
 5. Process for the production ofa fired ceramic, Fe₂O₃ comprising moulded body, with a test valueT_(0.5) according to DIN EN 993-8 (1997) of between 1400° C. and 1700°C., the process comprising: forming a moulded body using a compositioncomprising at least one purely magnesitic component and at least onecomponent containing CaO, all in a grain size of <8 mm and whichcomposition has the following oxidic analysis: a) 50 to 90% by weight ofMgO, b) 8 to 40% by weight of CaO, c) 1 to 8% by weight of Fe₂O₃, d) upto 10% by weight others, the total sum of a) to d) being 100%, andfiring the composition at a temperature >1400° C. which firing formsdicalciumferrite as a secondary phase.
 6. Process according to claim 5in which at least one CaO-containing component of the composition has agrain size of >2 mm.
 7. Process according to claim 5 in which at leastone CaO-containing component of the composition has a grain size of <5mm.
 8. Process according to claim 5 in which the purely magnesiticcomponent of the composition has a degree of purity of >90% by weightand a grain size of <5 mm.
 9. Process according to claim 5 in which thepurely magnesitic component of the composition has a degree of purityof >90% by weight and a grain size of <2 mm.
 10. Process according toclaim 5 in which the purely magnesitic component of the composition hasa degree of purity of >90% by weight has and a grain size of <0.3 mm.11. Process according to claim 5 in which the mean grain size (d₅₀) ofthe CaO-containing component of the composition is greater than the meangrain size (d₅₀) of the purely magnesitic component of the composition.12. Process according to claim 5 in which the grain size (d₉₅) of theCaO-containing component of the composition is greater than the grainsize (d₉₅) of the purely magnesitic component of the composition. 13.Process according to claim 5 in which at least one CaO-containingcomponent of the composition has a grain size of <1 mm.
 14. Processaccording to claim 5 in which at least one CaO-containing component ofthe composition has a grain size of <0.3 mm.
 15. Process according toclaim 5 with a Fe₂O₃ content of the composition of >1.5% by weight. 16.Process according to claim 5 with a Fe₂O₃ content of the compositionof >2% by weight.
 17. Process according to claim 5 with a proportion ofan MgO—CaO fused grain component in the composition.
 18. Processaccording to claim 5 in which the oxidic analysis of the compositionexhibits at least one of the following oxides: MnO, TiO₂, ZrO₂, SiO₂.19. Process according to claim 5, in which the purely magnesiticcomponent has a degree of purity of >90% by weight.
 20. Processaccording to claim 5, comprising lining a rotary kiln using the mouldedbody.